Reg:Impedance matching

[johnson.mike]

I want to match input impedance of a chip antenna (2.4-2.5GHz).
Below fig shows the layout.
I measured antenna impedance as 20.12-j82.6 Ohms and impedance of the input transmission line terminated with 50 ohm resistor at load end is 44.4-j12.3 Ohms.
I've calculated matching component values as ( seriesL=6.8nH and shunt C=1.6pF) using winsmith.
Using above values the total impedance is 20 Ohms.
What might be the problem?
Whether the calculated values in Winsmith was right?
Give suggestion for matching antenna impedance.

regards,
Johnson.

 

[hmsheng]

You should get an input impedance of 20.12+j*82.6 Ohm looking into the feed line from the antenna port after match. But with the data you given, the calculated input impedance after match with seris L and parallel C is 17.52-j*60.71 at 2.4GHz. It's different from both the target value and the 20 Ohm you measured. So the problem maybe in the impedance measurement process. Do you calibrate your equipment at 2.4GHz?

 

[johnson.mike]

hi hmsheng,

Thanks for your reply.
Yes, I've calibrated for the whole band 2.4 to 2.5GHz.
But the thing is it should be matched for 50 Ohms.ie.If u measure the impedance of the feedline it should be 50 ohms with antenna and matching circuit.
Can u suggest me the matching values for 50 ohm Matching? May the measurement process which we are following may not correct,Can you suggest me on how to proceed on this?

advance thanks.

 

[hmsheng]

No. You shouldn't match the DUT (device under test) to 50 Ohm when you measure it's impedance. Infact you can't do that because you do not know the load impedance before you measure it.
If your equipment, generally, VSA, is calibrated, you can measure the DUT directlly. Also you should note that the connecter will affect the measure result at 2.4GHz. If the connector is included in the calibration, the test result will be more accurate.

 

[johnson.mike]

No. You shouldn't match the DUT (device under test) to 50 Ohm when you measure it's impedance. Infact you can't do that because you do not know the load impedance before you measure it.
If your equipment, generally, VSA, is calibrated, you can measure the DUT directlly. Also you should note that the connecter will affect the measure result at 2.4GHz. If the connector is included in the calibration, the test result will be more accurate.

k then how to match to feed line impedance 44 Ohms

 

[hmsheng]

Step1, measure the input impedance of the feed line with 50 Ohm load at the other end, and get Zin;
Step2, measure the impedance of antenna, and get Zant;
Step3, match Zin to Zant*, done!

 

[johnson.mike]

In Above post I've mentioned the feedline impedance and antenna impedance.
Now can u suggest me the values of the matching elements.

 

[hmsheng]

With the load impedance of 44.4-j*12.3 Ohm and antenna impedance of 20.12-j*82.6 Ohm, the match network can be that in the attached figure with parallel C=1.24pF and seris L=7.04nH.

Please try this site:
http://www.circuitsage.com/matching/matcher2.html

You will find you can't get what you want with seris L first and then parallel C.